linux-kernel - Re: [PATCH] vfio: Fix build break when SPAPR_TCE

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Message-ID: <20161107092505.429f07d6@t450s.home>
Date:   Mon, 7 Nov 2016 09:25:05 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
        aik@...abs.ru, gwshan@...ux.vnet.ibm.com
Subject: Re: [PATCH] vfio: Fix build break when SPAPR_TCE_IOMMU=n

On Mon, 07 Nov 2016 19:34:42 +1100
Michael Ellerman <mpe@...erman.id.au> wrote:

> Paolo Bonzini <pbonzini@...hat.com> writes:
> > On 04/11/2016 06:48, Michael Ellerman wrote:  
> >> diff --git a/drivers/vfio/Kconfig b/drivers/vfio/Kconfig
> >> index da6e2ce77495..6b51a4ebed8a 100644
> >> --- a/drivers/vfio/Kconfig
> >> +++ b/drivers/vfio/Kconfig
> >> @@ -6,12 +6,12 @@ config VFIO_IOMMU_TYPE1
> >>  config VFIO_IOMMU_SPAPR_TCE
> >>  	tristate
> >>  	depends on VFIO && SPAPR_TCE_IOMMU
> >> -	default n
> >> +	default VFIO  
> >
> > No need to depend on VFIO since you already have it in "default".  

depends and defaults are different beasts though, if VFIO is not
enabled and we're not on a powerpc system with SPAPR,
VFIO_IOMMU_SPAPR_TCE should not be selectable, not just default to 'n'.

> 
> True, I can take that out.
> 
> > (I assume you cannot use "default y" because "depends on" doesn't downgrade
> > "y" to "m" when VFIO is a module.  
> 
> Correct.
> 
> > A shorthand is
> >
> > 	def_tristate VFIO && SPAPR_TCE_IOMMU  
> 
> Yep. My experience though is that a lot of folks don't really know what
> that means. So I prefer to spell it out with an explicit type, depends
> and default.
> 
> But I'll respin it that way if Alex prefers the shorter style.

Perhaps I'm one of those people.  Non-powerpc archs should not have an
option to select this, which is why the depends is there, AIUI.  So
long as we don't start exposing options that aren't relevant to a
platform, I'm flexible on what shorthands we use, but you may need to
teach me about them first.  Thanks,

Alex